1 files changed, 54 insertions, 0 deletions
diff --git a/src/parcom/plus.cr b/src/parcom/plus.cr
new file mode 100644
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+++ b/src/parcom/plus.cr
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+require "./parser.cr"
+
+module Parcom
+  # `Plus` is a parser that tries to parse with two different
+  # parsers in succession and fails if either of the two parsers fails.
+  # The return type of this parser is `{V, U}`, where `V` is the return
+  # type of the first parser and `U` is the return type of the second.
+  #
+  # Example:
+  # ```
+  # parse1 = Token.new('1')
+  # parse2 = Token.new('2')
+  # tokens = Tokens.from_string("12")
+  # result = (parse1 + parse2).parse(tokens) # succeeds
+  # result = (parse2 + parse1).parse(tokens) # fails, wrong order
+  # ```
+  #
+  # `Plus` parsers can be chained together, but the resulting return type
+  # can become unweildly with too many combined parsers:
+  # ```
+  # letter_a = Token.new('a')
+  # a5 = letter_a + letter_a + letter_a + letter_a + letter_a
+  # tokens = Tokens.from_string("aaaaa")
+  # a5.parse(tokens) # succeeds
+  # # `a5` has a return type of { { { {Char, Char}, Char}, Char}, Char}
+  # ```
+  #
+  # If you need to parse more than two things in this manner,
+  # consider using `Many`, `Some`, `Sequence`, or `TokenSeq` instead.
+  class Plus(T, V, U) < Parser(T, {V, U})
+    # Accepts the two parsers to use, in order.
+    def initialize(@p1 : Parser(T, V), @p2 : Parser(T, U))
+    end
+
+    # Tries to parse with the two given parsers, and returns
+    # their results in a tuple if the both succeed.
+    def parse(tokens : Tokens(T)) : Result(T, {V, U})
+      begin
+        r1 = @p1.parse(tokens)
+      rescue ex : ParserFail
+        raise ParserFail.new("Plus (left): #{ex.message}")
+      end
+
+      begin
+        r2 = @p2.parse(r1.tokens)
+      rescue ex : ParserFail
+        raise ParserFail.new("Plus (right): #{ex.message}")
+      end
+
+      Result.new(r2.tokens, {r1.value, r2.value})
+    end
+  end
+end
+